The backlight module of current LED advertising boards is usually made with a low pressure molding technology. The LED module made using this process has a beautiful appearance, high integration and fine waterproof properties. During using an LED module to make the backlight module of an advertising board, to obtain fine optical properties, usually an additional lens needs to be mounted at the LED module. To mount this type of lens, one manner disclosed in the prior art is that the lens is first mounted at a PCB board with an LED assembly and then the PCB board is placed in a mould in which a liquid plastic (hot) is perfused. However, in this process, the hot liquid plastic will damage the lens. Another known technical means is that an LED module has two covers which form a cavity for receiving a PCB board, and a lens is integrated at one of the covers and an opening is provided in the other cover for perfusing the liquid glue (not hot). During making, the liquid glue is injected into the cavity via the opening and is solidified therein. Although this method will not damage the lens, it uses too many components, with a high cost and complex assembling.